Recently, my team and I developed a declarative method to reassign Opportunities from a list view. Salesforce already had this capability for Cases, and we mistakenly assumed that the functionality existed for Opportunities. However, that functionality still hasn't been added as of Summer 18'. This is a simple solution that will hopefully save others a ton of time in maintaining clean assignments. It is a bit of a hack, but it works!

Opportunity owner is locked for editing within a list view. However, the owner can be updated with Process Builder. So, the workaround is to add another User based field that can be edited from the list view and allow Process Builder to perform the update in the background. Here is the summary, then screenshots.

Summary

Create "New Owner" lookup field
Update Owner to the New Owner value with Process Builder
Add the New Owner field to any Opportunity list view that supports Inline Edit

Screenshots

1. Create "New Owner" lookup field

Add a new lookup field to the Opportunity. The field should lookup to the standard User object.

Limit field level security accordingly and place the field on a page layout so it is available for use in a list view.

2. Update Owner to the New Owner value with Process Builder

Create a Process Builder flow that sets Owner = New Owner when New Owner is not null. Always clear the New Owner so it can properly trigger this change in the future.